GEOS 2 electric field observations during a sudden commencement and subsequent substorms

This paper presents and discusses electric field and plasma density data obtained by the GEOS 2 double probe experiment on March 22, 1979. The observation period encompasses a sudden storm commencement and two consecutive substorms. The E field data are compared with magnetic field recordings both at GEOS and on the ground. The influence of the solar wind on magnetospheric dynamics is studied with the help of additional data from IMP 8 and ISEE 3. The E field data, which were obtained when GEOS was in the dayside (front) of the magnetosphere, are grouped in transients, pulsations and underlying dc electric fields. We find that solar wind discontinuities cause strong electric field transients in the dayside magnetosphere. These transients are followed by Pc 4 oscillations of considerable amplitude. The data also show the existence of dc electric fields during buildup, onset and expansion phase of the two substorms. The combined observations by GEOS 2, ISEE 3, IMP 8 and ground-based magnetometers provide information on propagation of transients in the magnetosphere, on the nature of large scale oscillations and on the overall convection pattern during periods when the interplanetary magnetic field has a negative Bz component.